Television graphics are typically produced using a technique known as keying, which allows graphics and text typically created by CG (Character Generator) systems, such as those marketed by Chyron Corporation of Melville, N.Y., to be composited with or overlaid on video programming. The CG systems typically output two signals: a “key” signal and a “fill” signal. The “key” defines the outlines of the image data that is overlaid on the video, and the “fill” defines the contents that are inserted into the keyed area. Keyers are devices that receive as inputs (1) the key, (2) fill streams, and (3) video in baseband format, combine the three inputs, and output the result as a single composite video stream. Keyers can typically perform this function either with analog video or with uncompressed digital video. Keyers that perform this function with uncompressed digital video typically utilize SDI (Serial Digital Interface) as a transport mechanism. SDI describes the ITU R BT 601 standard for interface of component digital video.
FIG. 1 is a block diagram that illustrates keying of baseband (uncompressed) video. As shown in FIG. 1, the source video stream 105 onto which graphics are keyed is uncompressed, as are the fill stream 124 and the key stream 130 output by the character generator 120. The composite video output 115 from the video mixer/keyer 110 remains uncompressed.
FIG. 2 is a block diagram that illustrates keying of MPEG video converted to baseband (uncompressed) video. In situations where the source video is in a compressed format, such as an MPEG (Motion Picture Experts Group) format, it has been necessary for an MPEG decoder 240 decode the MPEG source video stream 235 to baseband (uncompressed) video stream 205 before keying. A Video Mixer/Keyer 210 applies keying to the uncompressed video stream 205, which is then re-encoded by an MPEG encoder 245 back to an MPEG format, to continue to the next stage of the workflow. Because existing keyers can work only with baseband video, the need for decoding and re-encoding imposes problems such as increased equipment cost, latency due to additional processing time required, and degraded video quality caused by decoding and re-encoding using standard lossy video compression algorithms.
Accordingly, a need exists in the art for an improved solution that enables the real-time keying of digital motion graphics on top of compressed video streams.